Simplify std::num::Primitive trait definition

This removes the `Primitive::{bits, bytes, is_signed}` methods and removes the operator trait constraints, for the reasons outlined below:

- The `Primitive::{bits, bytes}` associated functions were originally added to reflect the existing `BITS` and `BYTES` statics included in the numeric modules. These statics are only exist as a workaround for Rust's lack of CTFE, and should probably be deprecated in the future in favor of using the `std::mem::size_of` function (see  #11621).

- `Primitive::is_signed` seems to be of little utility and does not seem to be used anywhere in the Rust compiler or libraries. It is also rather ugly to call due to the `Option<Self>` workaround for #8888.

- The operator trait constraints are already covered by the `Num` trait.
Add FIXME comments regarding issue #11526.